In the ion implantation of semiconductor devices, a beam of energetic ions is directed at the surface of a semiconductor wafer. The ions are embedded in the wafer and provide regions of desired conductivities. The beam is typically scanned in one or two dimensions by electrostatic or magnetic deflection to produce a uniform distribution of ions over the wafer surface. The ion implantation system utilizes different ion species, different acceleration voltages and different beam currents for different processes. In addition, the system is required to scan different size areas, depending on the size of the wafer being implanted. In spite of the numerous variables, it is required that the ion implantation system supply an accurate ion dosage to the wafer with a high degree of spatial uniformity.
In order to accurately control the ion implantation process, it is desirable to know the instantaneous position of the scanned ion beam in a plane perpendicular to the nominal beam axis. The beam position information can, for example, be used in a feedback system to control scanning. The scan voltage applied to the deflection plates or to the scan magnets provides some indication of the beam position. However, due to the variability of the ion species, beam currents and beam energies and a number of other factors, it is difficult to accurately relate scan voltage to beam position. One technique for determining actual beam position is to locate a Faraday cup or sensing wire in the beam path. This technique interferes with the beam and must be utilized during a calibration mode. As a result, it slows down overall system speed and is impractical.
It is known to sense the position of an ion beam by placing sensing electrodes on opposite sides of the beam. Noise modulation on the ion beam induces voltages on the sensing electrodes. When the electrodes are symmetrically positioned with respect to the beam, the deviation of the beam from its nominal axis can be determined by processing the random signals induced on the two electrodes. This technique is described, for example, in (1) M. . Hodgart et al, "Remote Ion Beam Position Measurement From Random Beam Modulation," Inst. Phys. Conf., Ser. No. 38, 1978, Chapter 3, pp. 125-130; (2) W. J. Szajnowski, "Non-Destructive Techniques for Measuring the Parameters of Low-Energy Continuous Ion Beams," Proc., 4th Int. Conf. on Ion Implantation, Berchtesgaden, 1982; (3) W. J. Szajnowski, "Non-Destructive Ion Beam Position Monitors," ISIAT and IPAT Congress, Kyoto, September 1983; (4) W. J. Szajnowski, "Measurement of Ion Beam Parameters With Electrostatic Induction Electrodes," Vacuum, Vol. 34, Nos. 1-2, pp. 285-289, 1984; (5) W. J. Szajnowski, "Extraction Of Information On A Continuous Ion Beam From Beam-Induced Shot Noise," Nuclear Instruments and Methods in Physics Research, B6, 1985, pp. 176-182.
These references disclose functions for estimating the position of the beam based on the random signals induced on the two electrodes. The above-identified Szajnowski article in Vacuum Instruments and Methods discloses a maximum discloses a closed-loop scan generator for use in an ion implanter to precisely tailor scan patterns. The above-identified Szajnowski article in Nuclear Instruments and Methods discloses a maximum likelihood estimator for determining the position of an ion beam and also a sub-optimum estimator. The same article also suggests applications of the beam position measurement including alignment of the beam centroid, implementing an optimum scan pattern, stabilization of beam energy, monitoring the tilt angle of the beam's cross-section, monitoring the beam expansion during transport, and monitoring the stability and statistical properties of ion sources. A simple version of a log-ratio estimator without weighting has been described by V. N. Bringi et al in "Statistical Properties of the Dual-Polarization Differential Reflectivity (ZDR) Radar Signal," IEEE Trans. Geoscience And Remote Sensing, Vol. GE-21, No. 2, April, 1983, pp. 215-219 (see equation (8)).
One source of error in practical dual-channel signal processors is the mismatch between channels of both amplitude and phase response. The two channels can be equalized to some extent by trimming. However, trimming over a several megahertz frequency band is difficult. It is well known that the instrumentation error due to channel mismatch can be reduced by channel switching (see, for example, M. Sachidananda et al, "ZDR Measurement Considerations For a Fast Scan Capability Radar," Radio Science, Vol. 20, No. 4, T pp. 907-922, July-Aug. 1985, FIG. 16b). However, the instrumentation error due to channel mismatch is not entirely eliminated when used with prior art signal processors.
In order for the above-described beam monitoring technique to be practical for real time monitoring of beam position in commercial ion implantation systems, it must meet certain requirements. These requirements include a high accuracy for a variety of ion species, for beam currents typically in the range of 0.1 microampere to 30 milliamperes, for beam energies typically in the range from 10 keV to 400 keV, for scan areas up to 8 inches square and for scan frequencies typically up to 1,000 Hz. The sensor must be able to accurately reproduce the beam position as a function of time under all the above conditions.
In prior art beam position sensing techniques, errors have arisen from a variety of sources including:
(1) Gain mismatch between the two processing channels. Gain is difficult to match in both magnitude and phase over the required frequency band of a few megahertz.
(2) The nonstationary random characteristics of a beam-induced signal result in fading of the signal and short spikes exceeding the dynamic range of an amplifier.
(3) Analog implementation of signal processors perform the required difference/sum normalization utilizing divider circuits, automatic gain control and logarithmic conversion. All analog circuits implementing these functions are not precise and require adjustments.
(4) Prior art techniques cannot accurately track the time varying position of the rapidly scanned beam because of the phase distortions of the reconstructed beam path. These are dynamic errors.
The requirement for processing two random signals and determining the relative values thereof has been described above in connection with determining the position of a scanned ion beam in an ion implanter system. Equivalent requirements arise in other technical fields. For example, an ion beam position sensor can be utilized in a high energy accelerator to determine the position of various charged particle beams. Similar signal processing requirements arise in connection with optical position sensors, weather radar for rainfall estimation, monopulse radar systems and sonar systems for angle-of-arrival estimation, radio astronomy systems for tracking a radio source, and detection of projectiles by electric field measurements. In general, the signal processing requirements arise in connection with any measurement system for computing difference/sum ratios of two random signals corrupted by noise.
It is a general object of the present invention to provide improved signal processing apparatus.
It is another object of the present invention to provide signal processing apparatus for determining the relative values of two random signals with extremely small error.
It is still another object of the present invention to provide signal processing apparatus which when utilized with channel switching, completely eliminates errors due to channel mismatch.
It is yet another object of the present invention to provide signal processing apparatus utilizing weighted integration of log-ratios where the weighting is adaptive and depends upon the values of signal samples being processed.
It is a general object of the present invention to provide improved apparatus for real time monitoring of the position of an ion beam relative to its axis.
It is a further object of the present invention to provide apparatus for accurately monitoring the position of an ion beam for different beam currents, beam energies and ion species.
It is a further object of the present invention to provide apparatus for accurately monitoring the position of a rapidly-scanned ion beam.
It is yet another object of the present invention to provide apparatus for monitoring the position of an ion beam without interfering with the beam.